The role of earthworms in organic matter cycles

Abstract

Plant organic material that reaches the soil is subject to many decomposing agents, including both micro-organisms and animals. Very soft plant and animal residues may be decomposed by the microflora but much organic matter, particularly the tougher plant leaves, stems and root material, does not breakdown without first being disintegrated by soil animals and acted upon by enzymes in their intestines. Earthworms have an important role in this initial process of the cycling of organic matter, because a few common species such as L. terrestris seem to be responsible for a large proportion of the fragmentation of litter in woodlands of the temperate zone. Soils with only few earthworms often have a well- developed layer of undecomposed organic matter lying on the soil surface. Edwards and Heath (1963) showed that in two sites earthworms consumed more oak and beech litter than all the other soil invertebrates together. In apple orchards L. terrestris removed more than 90% of the autumn leaf fall, during the course of the winter; this was calculated to be about 1.2 tonnes of dry weight of leaves per ha of orchard (Raw, 1962) (Fig. 46). The effectiveness of L. terrestris in initiating the fragmentation and incorporation of fallen apple leaves was vividly illustrated by comparing the soil profile and structure of an orchard with a large L. terrestris population, with one in which earthworms were almost totally absent (due to frequent and heavy spraying with a copper-based fungicide). The orchard with few earthworms had an accumulated surface mat, 1–4 cm thick, made up of leaf material in various stages of a very slow decomposition and sharply demarcated from the underlying soil, which had a poor crumb structure (Plate 8).